A 3-year longitudinal clinical research project is proposed to study the incidence of root surface caries and the microbial flora associated with the initiation of lesions. The elderly population at Parkwood Hospital, London, Ontario, will be studied. A preliminary study of root caries prevalence in this population indicated that a sufficient number of new lesions should develop. Each of approximately 50 subjects will be examined annually to record 1) plaque accumulation and periodontal inflammation indices; 2) distribution of developing root lesions; 3) distance of lesions from the crest of gingival margins; and 4) gingival recession. Bitewing radiographs will also be taken. An average of 8 root surface samples and a saliva sample will be collected from each subject three times annually. The samples will be diluted and plated on non-selective and selective media. After transportation of plates to Toronto, the proportions and isolation frequency of potentially cariogenic bacteria in the total cultivable flora will be determined, with special reference to streptococcus mutans, Actinomyces viscosus, Actinomyces naeslundii, lactobacilli, enterococci, and Arthrobacter-like microorganisms. A bacterial flora profile will be incorporated in the general computer analysis to determine the relationship of the degree of odontopathic infection and the clinical parameters measured. The degree of association of both bacterial and clinical parameters with the initiation of root lesions will be calculated. The data should indicate the microbial and periodontal environments which determine susceptibility to root caries or maintenance of root surface health.